T lymphocytes play a central role in almost all immune responses. Their receptors are stimulated by a composite ligand formed by binding of processed fragments of protein antigens to Major Histocompatibility Complex (MHC) glycoproteins. The class I MHC molecules present antigens to T lymphocytes expressing the CD8 glycoprotein and generally of cytotoxic function. In human populations the polymorphic differences between these HLA-A,B,C molecules provide the antigenic target for alloreactive cytotoxic T cells that contribute to rejection of transplanted tissues. The overall goal of this research is to relate the molecular mechanisms of HLA-A,B,C function to their primary and three dimensional structures. A dissection of the interaction of HLA-A,B,C with the CD8 glycoprotein provides one specific aim. A new assay for CD8-class I MHC binding will be used to analyze the natural polymorphism in class I molecules and site. Preliminary results show that HLA-A28 is a natural molecule that does not bind CD8. This unexpected polymorphism will facilitate the proposed analysis and raises questions as to the selective advantage of such a molecule. A major challenge to the rigorous molecular analysis of class I MHC function is the development of biochemical assays for peptide binding and our second specific aim is directed to this goal. Approaches using purified molecules and cells will be investigated. manipulation of beta2-m association is hypothesized as one route to destabilize the structure and increase the availability of the peptide binding site. preparations of class I molecules that are enriched for a chosen peptide will be used to make HLA restricted antibodies. These reagents will be invaluable in studying the intracellular association of class I molecules with peptides. The peptide binding function and central immunological role of HLA- A,B,C has resulted in various pathogens divisively using these molecules. Two examples to be studied under aim 3 are adenovirus which has a protein (E3/19) that traps HLA-A,B,C inside the cell and cytomegalovirus that disguises itself in a coat of beta2-m- microglobulin. By analysis of the mechanisms involved we hope to gain greater insights into HLA-A,B,C function and an understanding of the viral subterfuge.